This invention relates to servo systems and in particular to a digitally controlled servo loop for maintaining constant speed in a brushless DC motor, and for stopping the rotor at a particular angular orientation.
DC motors are manufactured which have a rotor element including permanent magnets having a plurality of successively spaced north and south poles. The permanent magnet is disposed proximate a fixed stator element having a plurality of windings or coils for producing magnetic fields when current is conducted therein. By selectively energizing the coils, they generate magnetic fields which impart forces on the magnetic fields emanating from the poles of the rotor magnet to produce rotation of the rotor. Commonly, Hall effect sensors are located in fixed relation to the coils and disposed proximate the rotor magnet to sense the relative position of the poles on the rotor magnet. Signals from the Hall sensors are employed to phase the energization of the coils to produce efficient operation of the motor.
It is known to control the speed of such motors by comparing the rate of rotation thereof to a known frequency such as from a stable oscillator and adaptively adjusting the motor drive parameters to eliminate the frequency error therebetween. Typically the output of such control or servo systems are applied to adjust either the bias potential of the commutating Hall sensors or the supply potential to the coil driver.